Will you be able to check in for your flight that early? Should be, but you might want to confirm. Then you'd have just your carry-on bag(s) with you; hopefully small. Look at any guide to DC and see where you want to go, then look at www.wmata.com for Metro schedules. Remember that the trains can get crowded at rush hour, and you should be back at the airport an hour before your flight, since security lines can be long. Really not a lot of time.